Involvement of proline in protecting thylakoid membranes against free radical-induced photodamage

Investigations were carried out to determine the involvement of proline in protecting thylakoid membranes against photodamage. The exposure of thylakoids isolated from the cotyledons of Brassica juncea to photoinhibitory light caused a linear increase in lipid peroxidation with time as measured by the assay of conjugated dienes and malondialdehyde (MDA). Proline produced a considerable reduction in the lipid peroxidation-linked formation of both conjugated dienes and MDA in the thylakoids during exposure to strong light. This reduction in lipid peroxidation reflects the probable involvement of proline in the reduction of the free radicals generated on strong light exposure. The thylakoids exposed to strong light intensity also generated singlet oxygen (O-1(2)) and superoxide radical anion (O-2(-)). The level of these radicals increased with the time of exposure. The presence of proline significantly curtailed the production of O-1(2), by the thylakoids during strong illumination. However, the presence of proline did not affect the O-2(-) induction capacity of the thylakoid membranes during strong light exposure. Peroxidation of linolenic acid micelles occurred on exposure to UV radiation. The almost total suppression of UV-induced lipid peroxidation of linolenic acid micelles in the presence of sodium azide and L-histidine (singlet oxygen quenchers) suggested the involvement of O-1(2) in UV-induced lipid peroxidation. The presence of proline during the exposure of linolenic acid micelles to UV radiation caused a considerable reduction in the formation of conjugated dienes as well as MDA. The proline-induced reduction in the peroxidation of these micelles further confirms the involvement of proline in reducing the level of O-1(2) Our results therefore demonstrate that proline is involved in reducing the photodamage in the thylakoid membranes by scavenging and/or reducing the production of O-1(2). (C) 1997 Elsevier Science S.A.